The present invention relates to organic light emitting diode (OLED) displays having a plurality of pixels, and more particularly, to top-emitting displays that include an auxiliary electrode for improving the conductivity of a transparent continuous electrode in the display.
Flat-panel displays, such as organic light emitting diode (OLED) displays, of various sizes are proposed for use in many computing and communication applications. In its simplest form, an OLED is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. OLED displays may be constructed to emit light through a transparent substrate (commonly referred to as a bottom emitting display), or through a transparent top electrode on the top of the display (commonly referred to as a top emitting display).
Materials for forming the transparent electrode of top emitting displays are well known in the art and include transparent conductive oxides (TCO""s), such as indium tin oxide (ITO); thin layers of metal, such as Al, having a thickness on the order of 20 nm; and conductive polymers such as polythiophene. However, many electrode materials that are transparent, such as ITO, have low conductivity, which results in a voltage drop across the display. This in turn causes variable light output from the light emitting elements in the display, resistive heating, and power loss. Resistance can be lowered by increasing the thickness of the top electrode, but this decreases transparency.
One proposed solution to this problem is to use an auxiliary electrode above or below the transparent electrode layer and located between the pixels, as taught by US2002/0011783, published Jan. 31, 2002, by Hosokawa. The auxiliary electrode is not required to be transparent and therefore can be of a higher conductivity than the transparent electrode. The auxiliary electrode is typically constructed of conductive metals (Al, Ag, Cu, Au) that are also highly reflective. This results in incident light reflecting off the auxiliary electrode and thereby reducing the overall contrast ratio of the display. This makes the display less effective for use under high ambient light conditions, such as outdoors under sunshine.
There is a need therefore for an improved top emitting OLED display that uses an auxiliary electrode and has improved contrast.
The need is met according to the present invention by providing a top emitting OLED display that includes a substrate; a patterned electrode formed above the substrate, defining a plurality of light emitting elements having gaps between the light emitting elements; a layer of OLED material disposed above the patterned electrode; a continuous transparent electrode disposed above the layer of OLED material; and a light-absorbing auxiliary electrode that is thermally and electrically conductive and in electrical and thermal contact with the continuous transparent electrode and located over the gaps between the light emitting elements of the display.
This invention has the advantage over other top emission devices having auxiliary electrodes of improved contrast and heat dissipation and therefore improved usability in bright ambient conditions such as in sunlight